Questions about the second day 3 video

ISG Workshop 2026 Day 3
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I have some questions about the second Day 3 video, “Estimate heritability using data from relatives/twins”:

  1. On slide 28 (roughly 19:34 in the video), it compares saturated and constrained models. How can a constrained model (m2) have a better fit (lower AIC) than a saturated model, wouldn’t a saturated model always have the best fit?
    • I realize the difference isn’t significant according to the LRT, but I’m curious how it’s possible for the constrained model to have even a slightly better fit.
  2. On slide 29 (21:49 in the video) it talks about calculating maximum likelihood correlations based on the variances and covariances from fitting the model. Is there ever a reason why you would want to do this (vs. just calculating the correlations directly)? Are there times when the maximum likelihood correlations might differ from the directly-calculated correlations in a way that tells you something important about the data?
  3. Question 3 about model violations has been moved to its own thread
  4. Question 4 about dominance has been moved to its own thread

Thanks for all the great videos, and sorry to ask so many questions!!